This invention relates to pneumatic clutches, and particularly to a mechanism for protecting such clutches from potentially damaging overload conditions.
Pneumatic clutches find use in many applications and are particularly used in marine engine drives. The clutches connect an engine to a gear drive which in turn drives a ship's propeller. Typically, there will be one pneumatic clutch for ahead direction and another for astern direction of travel.
The common form of pneumatic clutch uses an inflatable annular gland. The typical gland-type clutch has a series of clutch shoes mounted on the inner periphery of the gland. Air under high pressure is admitted to the interior of the gland to expand it thereby bringing the clutch shoes into engagement with clutch drums. The glands are typically manufactured from rubber with reinforcement plys.
Torque is transmitted by the friction developed between the clutch shoes and the clutch drums. The amount of torque that the clutch can carry is dependent on the physical size of the clutch, the air pressure, and the coefficient of friction between drum and shoes.
The marine vessels on which these clutches are used are often operated in rivers in which floating debris is routinely encountered. Occasionally the propellers become jammed due to a log or other debris being caught between the propeller and the ship's hull or to a steel cable being wrapped around the propeller. When such a jam occurs, a high shock load results due to the inertia of the system and the energy of the engine. The shock and energy is reflected in the clutch and, as a result, the shoes tend to slip on the drum. When this happens, a large amount of heat is generated between the clutch shoes and the drum and thermal stresses are introduced into the drum which can lead to failure of the drum.
This invention concerns a mechanism to release the clutch when an overload condition occurs so as to prevent destruction of the clutch elements. The mechanism relies on a rotational stiffness constant between the clutch driving and driven members.